The invention relates to a polyurethane lacquer which is suitable , in particular, for the high-gloss coating of polystyrene, said lacquer being on the basis of polyhydroxyacrylate and polyisocyanate and containing a catalyst and a tertiary alcohol.
The invention also relates to a spray diluent for such a polyurethane lacquer.
The invention further relates to a method of lacquer spraying polystyrene-containing substrates, using a polyurethane lacquer, containing a polyhydroxyacrylate, a polyisocyanate, a catalyst and tertiary alcohol.
Industrial applications of polystyrene are, among others, supports for household appliances, housing for audio and video equipment, shavers and wheel covers for cars. Polystyrene is often used in the form of so-called high-impact polystyrene, in which a few percent of rubber are mixed with the polymer. For many of these applications it is required that the surface of the polystyrene is provided with a hard, properly adhering decorative coating. This coating can be obtained by applying a lacquer, preferably, by spraying.
Physically dryable lacquers are not very attractive because of their high content of organic solvents. Owing to tightened environmental requirements the emission of solvents has to be minimized. Also since there is no crosslinking in physically drying lacquers, the resistance to wear and the chemical resistance of the coating manufactured by means of these lacquers are often insufficient.
Lacquers which are more suitable are two-component lacquers, for example polyyrethane lacquers, which are cured by means of crosslinking and which contain relatively few organic solvents.
Polystyrene is attacked by many common solvents, such as ketones and ethers, so that an object made from polystyrene is subject to deformation and the surface is attacked in such a way that it attains an undesirably mat appearance.
A polyurethane lacquer on the basis of polyhydroxyacrylate and polyisocyanate is known from European Patent Application no. 1304. The lacquer described therein contains in addition to a tertiary alcohol also 18-60% by weight of one or more other solvents such as ketones or ethers. This lacquer is unsuitable for use on polystyrene, because the latter solvents attack the polystyrene, which leads to the above-mentioned disadvantages. Additionally the lacquer described therein contains no adhesion-improving substance so that for many applications a primer has to be provided first on the substrate, which implies an additional process step. This is generally referred to as a two-layer paint system.